1. Field of the Invention
The present invention relates to a process for producing tyres for vehicle wheels, to the tyres thus obtained and to the crosslinkable elastomeric compositions used therein. More particularly, the present invention relates to a process for producing tyres for vehicle wheels, which can be carried out in the substantial absence of conventional crosslinking agents, to the tyres thus obtained and to the crosslinkable compositions used therein.
2. Description of the Related Art
Processes for vulcanizing diene elastomers with sulphur are widely used in the rubber industry for the production of a wide range of products, and in particular tyres for vehicle wheels. Although these processes give high-quality vulcanized products, they are considerably complicated to carry out, mainly due to the fact that, in order to obtain optimum vulcanization within industrially acceptable times, it is necessary to use a complex vulcanizing system which includes, besides sulphur or sulphur-donating compounds, one or more activators (for example stearic acid, zinc oxide and the like) and one or more accelerators (for example thiazoles, dithiocarbamates, thiurams, guanidines, sulphenamides and the like). The presence of these products can, in some cases, entail considerable problems in terms of the harmfulness/toxicity both during production and during use, in particular when the vulcanized products are intended for medical/health-care or food use. In addition, it is known that the use of sulphur or sulphur-donating compounds leads, during the vulcanization stage which is generally carried out at temperatures above 150° C., to the development of volatile sulphurized compounds.
Consequently, in recent years, research efforts have been directed along two different lines, the first being to improve the known vulcanization processes in order to make them more efficient and cleaner, the second aimed at developing alternative crosslinking techniques. Although appreciable progress has been made, it is not possible to state at the present time that alternative techniques to crosslinking with sulphur exist which would give similar results and would simultaneously afford an effective simplification in terms of production. For example, crosslinking processes via peroxide compounds require special precautions on account of the instability of these compounds, in addition to requiring the use of activators. Crosslinking by means of radiation involves the use of complex equipment, as well as the incorporation of all the precautions required when high-energy and high-power radiation is used.
So-called “self-crosslinking” elastomeric compositions, i.e. compositions which do not require the use of crosslinking agents such as sulphur or sulphur compounds, are known in the art.
For example, U.S. Pat. No. 2,724,707 describes elastomeric compositions consisting of a diene polymer containing carboxylic groups, in particular a carboxylated nitrile rubber (XNBR) obtained by partial hydrolysis of a butadiene/acrylonitrile polymer, in which a polyvalent metal oxide (for example zinc oxide) is dispersed. On heating these compositions, they crosslink according to a mechanism of ionic type.
A study of the crosslinking of XNBR with a high degree of carboxylation, by reaction with an epoxy resin (for example bisphenol A diglycidyl ether) in the presence of reinforcing fillers such as carbon black, silica and clay, is reported in the article by S. K. Chakraborty and S. K. De, published in the Journal of Applied Polymer Science, Vol. 27, pp. 4561–4576 (1982). The crosslinking is carried out by heating the compound to 150°–180° C. As is known, epoxy resins are low molecular weight products in which the epoxide (or oxirane) groups are “external”, i.e. they are located in a terminal position on the main hydrocarbon chain, the oxygen atom forming the oxirane ring being linked to both the last and the penultimate carbon atoms of this chain.
A study of the crosslinking of a composition based on epoxidized natural rubber (ENR) and XNBR is reported in the article by R. Alex, P. P. De, N. M. Mathew and S. K. De, published in Plastics and Rubber Processing and Applications, Vol. 14, No. 4, 1990. In particular, this article describes the crosslinking of compositions consisting of ENR and XNBR as such or containing silica or carbon black as reinforcing filler. According to what reported by the authors, the crosslinking reaction in the ENR and XNBR mixtures comprises the formation of ester bonds between the epoxide groups and carboxylic groups. The rheometric curves would show absence of reversion, stability of the crosslinked structure and a high rate of crosslinking.
Italian patent IT 1,245,551 describes self-crosslinking compositions containing an epoxidized elastomer and a crosslinking agent of formula R1-R—R2, in which R is an arylene, alkylene or alkenylene group, while R1 and R2 are carboxylic, amine, sulphonic or chlorosulphonic groups. Dicarboxylic or polycarboxylic acids, or mixtures thereof, can be used as crosslinking agents. Self-crosslinking compositions containing an epoxidized elastomer and a second elastomer in which the repeating units of the polymer chain contain at least one carboxylic group are also described. For example, self-crosslinking compositions are obtained by mixing an epoxidized elastomer (for example the products ENR 25 or ENR 50 which are available under the brand name Epoxyprene® from Guthrie Symington Ltd.) with a butadiene/acrylic acid copolymer (for example a product sold by Polysar/Bayer under the brand name Krynac®). The crosslinking reaction takes place by heating between the epoxide groups and the carboxylic groups, with formation of ester bonds.
U.S. Pat. No. 5,173,557 describes self-vulcanizing compositions comprising an elastomeric polymer functionalized with isocyanate groups and a compound containing at least two active hydrogens of Zerewitinoff type, or self-crosslinking compositions comprising an elastomeric polymer containing active hydrogens of Zerewitinoff type and a compound containing at least two isocyanate groups. Alternatively, an elastomeric polymer containing either isocyanate groups or active hydrogens of Zerewitinoff type can be used, without using an additional crosslinking agent. The active hydrogens can be present, for example, on hydroxide, amine, carboxylic or thiol groups. In order to avoid undesired pre-crosslinking of the elastomer, the isocyanate groups are blocked beforehand with suitable functional groups, which are removed by heating before the crosslinking reaction between the free isocyanate groups and the active hydrogens, optionally with the aid of a catalyst.
On the basis of the Applicant's experience, the self-crosslinking compositions proposed hitherto in the prior art do not provide a valid alternative to conventional compositions vulcanized with sulphur or derivatives thereof. The reason for this is that the performance qualities of the crosslinked products are generally unsatisfactory, in particular for applications such as tyre compounds, in which a substantial consistency of the elastic performance qualities over a wide range of working temperatures and at the same time high abrasion resistance without unacceptably increasing the hardness is required. This is the case, for example, for the self-crosslinking compositions described above in which a polymer containing carboxylic groups (for example XNBR) is hot-crosslinked in admixture with an epoxidized elastomeric polymer or with an epoxy resin.